There are plenty of great options: Giggs, Scholes, Baresi, Maldini, Facchetti, Puyol, etc.
For me though, the greatest is Francesco Totti. The others above played for clubs who won multiple trophies, and all won the champions league at least once. Aside from a smattering of Coppa Italias and one Scudetto, Totti didn’t win anything with Rome.
For me, it’s easier for a top player to stay at a top club, rather than be a top player like Totti and stay at a club that was rarely considered the best in Serie A. For my money, he is the greatest one-club man of all time.
Who would you choose?
Michael Zorc. Born in Dortmund. Played for a different club in Dortmund in his youth, but then joined BVB when he was 16. 17 years in the senior side, 463 games, 131 goals as a midfielder. 9 years as captain. CL winner, two-time Bundesliga winner, DFB Pokal winner, Club World Cup winner, three-time German Supercup winner.
AND THEN he becomes Sporting Director of the club four years after retiring as a player. Leads the team back out after the near bankruptcy at the beginning of his tenure (not his fault) and back to (inter)national glory. Two more Bundesliga titles, three more Pokals, three more Supercups and a CL final. Retires in 2022 after 37 years at the club.